1. Field of the Invention
This invention relates to wheel alignment measurement and more specifically to the alignment of steerable vehicle wheels which are positioned at certain angular relationships known as toe and camber.
2. Description of the Prior Art
The procedure to which this invention relates is one wherein the front, or steerable, wheels of a vehicle have mounted thereon a fixture, or clamp, for use in the alignment measurement procedure. The fixture commonly employed is clamped to the rim of the wheel by means of clamps mounted on opposite ends of a cross bar structure. Adjustably mounted on the cross bar structure is a slide, which can be considered to correspond to the hub of the wheel. In the embodiment of the fixture disclosed herein, the slide has a stub shaft projecting therefrom which theoretically represents an extension of the wheel spindle. Pivotally mounted on the stub shaft is a counterweighted support or bracket, which serves the purpose of mounting angle measuring instruments, including sensors or detectors for providing information as to the inclination of the wheel axis of rotation in the horizontal plane (toe) and in the vertical plane (camber). Of course, the plane of rotation of the wheel is 90.degree. to the axis of rotation of the wheel.
In the fixture shown herein, sensors are supported on a bracket that is pendulously mounted on the aforesaid stub shaft of the slide so that if the wheel is jacked up to clear the ground and rotated, the sensor support bracket does not rotate with the wheel. Of course, a reversed pendulous mounting structure could be provided wherein the sensor bracket has a stub shaft that is rotatably fitted into a socket in the slide, or simulated hub of the fixture.
Ideally, the aforesaid stub shaft of the fixture, or its equivalent, should be parallel to the axis of rotation of the wheel, which corresponds to the spindle axis. Under these ideal conditions, the angular disposition of the stub shaft or sensor mounting means does not change in either the vertical or the horizontal plane when the wheel is rotated. Thus, the inclination angles measured in the vertical and horizontal planes would be the true measurements of the inclination of the wheel rotational axis in those planes. However, if the axis of the stub shaft is inclined from the axis of wheel rotation (the spindle axis) and hence is not perpendicular to the plane of rotation of the wheel, a condition known as "run-out" is presented. Run-out causes the sensor stub-shaft axis to gyrate as the generatrix of a cone and thus produces an angular inclination of the stubshaft axis with respect to a reference plane which varies from the true inclination of the spindle axis with respect to that plane as the wheel rotates. Such angular deviation in the vertical plane is a run-out error in camber and a corresponding angular deviation of the stub shaft axis in the horizontal plane is a run-out error in toe. Run-out can occur either because the fixture is clamped onto the wheel rim so that the axis of its stub shaft or equivalent is not parallel to the spindle axis or because the wheel rim itself is not in a plane perpendicular to its axis of rotation. Either or both of these conditions will produce run-out.
The U.S. Pat. No. to Senften 3,892,042, issued July 1, 1975, recognizes that run-out will introduce errors in the measurement of toe and camber angles of the vehicle wheel when employing a fixture of the general type previously mentioned. The solution proposed by the aforesaid Senften patent requires the rotation of the wheel into two precisely predetermined positions that are exactly 180.degree. apart, coupled with the generation of separate and discrete inclination signals at each of said two positions. Inclination measurements are made for both toe and camber, but in either plane such measurements must be obtained at each of two wheel rotational positions which are exactly 180.degree. apart. To obtain the true inclination of the axis of rotation of the wheel in each of the desired planes, Senften adds the two discrete and individual inclination readings taken exactly 180.degree. apart and divides them by two. The result of this operation cancels out the effect of run-out and hence gives the true inclination of the wheel axis relative to the desired plane, such as either the vertical or the horizontal plane. No measurement of the maximum run-out error (which may occur at any angular wheel position) is provided by this method.
In order to provide optimum accuracy when employing the run-out compensation method of the Senften patent, it is essential that measurements be taken exactly 180.degree. apart, and, therefore, the position of the switch feelers 33, 34 relative to the operative edges of the slots 24-26 in the disc 23 of the Senften apparatus must be precisely adjusted or positioned, and the precise point of operation of the switches must be maintained during the life of the apparatus.